Charged particle treatment of substrates is used in a variety of applications. One specific application is ion beam etching of substrates having features with extreme critical dimension uniformity and symmetry requirements, in particular, “pole trimming” and “pole slimming” processes used in formation of a thin film magnetic head for a disk drive. In such processes, the substrate, depending on the particular application, is positioned at one or more angles to the beam, between normal incidence (0°) and glancing incidence (e.g. about 70°). Depending on the application, the substrate may be required to be held statically at one or more fixed angle(s), or may be moved in front of the beam (e.g. tilted and/or rotated) during a single substrate process cycle to improve the substrate treatment uniformity.
Shortcomings in present charged particle sources will limit the use of such processes. The reduction of critical dimensions of thin film devices, according to the general trend of semiconductor and data storage device miniaturization, has increased the need for process uniformity, and better control of beam collimation. At the same time, reduction in device size has required better control of device critical dimensions (“CD”s) that are a function of the ion bombardment process, such as the average wall angle of an etched structure. Equally important, variations in these CDs need to be better controlled.
As a result, for modem, critical applications, very high process uniformity (e.g. etch depth) may be required, to within 1 to 3%. Uniformity, however, is directly related to the particle flux, i.e. the beam current density, profile, and the angular distribution of the charged particles at the substrate is directly related to the angular properties of the beamlets extracted from the plasma by the optics of the source. These requirements dictate that the angular impact of the particles across the substrate should be, with very small tolerance, in exactly the same direction, with small and uniform local angular spread, and the flux of these particles (plus the charged particles converted to energetic neutrals during the beam transport) to the substrate should be at the same time very uniform. For the charged particle beam, this translates to a more directional, parallel, and simultaneously more uniform current density broad beam. For example, an ion beam treatment system is required in which the average ion divergence angle is <3 to 5°, and both the beam divergence angle and beam parallelism are controlled within <0.5 to 1° deviation.
At the same time, there is a need for improved “static etch” charged particle treatment processes, in which a substrate is held at a fixed angle with respect to the beam and must therefore be uniformly treated over a large volume of beam space. Large area static etch applications have been developed relatively recently, driven by special needs such as ion beam “pole tip thinning” of thin film magnetic heads. In these processes, the substrate cannot be rotated or moved to “average out” variations in the ion flux across a substrate. Since the substrate may be held at a steep angle to the ion beam, the ion flux must be uniform in the entire three-dimensional space containing the substrate surface. Although prior designs seek to control process uniformity, divergence angle and parallelism, current charged particle sources can usually only be optimized for one of these parameters, by compromising another.
For example, some known techniques for achieving highly uniform particle flux processes use conventional plasma-driven particle sources combined with substrate motion (such as planetary motion) to average out variations in the beam flux uniformity; these techniques do not provide sufficient control of the directionality, parallelism, or uniformity of the angular distribution of ion incidence across the substrate surface, but rather, for practically limited beam dimensions, tend to aggravate these problems by moving the substrate outside of the most parallel region of the beam.
Another known approach to improved particle flux uniformity in a charged particle processing system, is to form charged particle beamlets in a certain pattern by specific design of the charged particle optical system used to extract the charged particles from the generator, said optical system, for example, comprising a set of electrodes having multiple extraction apertures, the location, size and density of which can be adjusted across the beam extraction area. Such a grid assembly, operated under proper conditions, is also capable of achieving the desired device features across small areas of a substrate. However, the beam flux uniformity and charged particle angular distributions generated by conventional grid assemblies have unacceptable variations, due to variations in mechanical tolerances of the grid assembly and thermal expansion effects under service conditions. Furthermore, a low beam divergence charged particleoptic design does not compensate for variations in the charged particle generator, which can themselves affect the beam collimation and uniformity. In fact, low beam divergence requirements result in greater sensitivity of the beam profile on the substrate to variations in the particle source, since beam divergence tends to average out such variations downstream from the ion source.
Thus, advanced charged particle designs having very tight control of grid structure dimensions, and thermal expansion effects, when combined with a conventional particle generator, are still not able to meet the divergence uniformity requirements or beam parallelism at the substrate across large diameters, due to limitations of the generator.
As mentioned above, it is known in the art that the charged particle flux uniformity, beamlet directionality, and beam divergence uniformity can be partially controlled by the design of the gridded optics. By patterning the individual grid hole sizes or hole density in different zones across the grid area, for example, the beam current density uniformity can be improved. However, there are practical limitations to the efficacy of these techniques. One is that changes in one zone, can, due to beam divergence, affect other areas of the substrate not directly within the same zone. Also, the effectiveness is lost if very small changes on the orders of the mechanical tolerances are required. Furthermore, the procedures which have been used in the past for practical beam etching systems, commonly known as “masking”, “zoning” or “blocking” techniques, were specifically developed for use with substrate rotation to average out azimuthal variations in beam density. These techniques are unsuited for static substrate treatment processes.
With regard to the objective of improving the plasma uniformity or ion current density uniformity across a surface within a plasma-driven particle generator, a number of attempts have been described in the literature of radio frequency (RF) Inductively Coupled Plasma (RF ICP) sources. This includes: (1) a helical rf coil design with movable re-entrant plasma shaping plug with or without accessory magnetic fields; (2) “pancake”-type planar rf coil generator designs equipped with modified geometries of the rf coil; (3) planar or domed rf coil generator designs equipped with independently controllable multiple rf coils; (4) a planar rf coil generator design equipped with magnetic-pole-confinement; (5) a helical resonator rf plasma source with optimized rf tap. Pancake-type plasma generators are not very suitable for practical ion beam sources, due to deposition of ion beam sputtered material on the “rf window” area, causing the need for frequent maintenance. The other potential solutions are complicated and expensive.
Thus, there is a need for a charged particle source for treating a substrate with reduced angular dispersion of the charged particles across a large substrate, while also maintaining high uniformity across the substrate at any angle to the beam.
A second, independent need is to improve uniformity of a charged particle treatment process. An exemplary process where this need will arise, is ion beam etching of smooth surfaces, or substrates with devices without very challenging critical dimensions, such as a seed layer etch process. There, improved uniformity of treatment can provide better device yields.
A third need is to improve processes in which the substrate is held statically at an angle during the process.
There is a further need for a charged particle source that is “tunable” to compensate for variations due to part tolerances as well as changes in source performance with time.